Bistable liquid crystal device having two drive modes

ABSTRACT

A twisted nematic bistable liquid crystal ( 2 ) switching between two stable states in a high voltage mode is used in an AMLCD low voltage drive. The picture electrodes ( 14 ) and the counter electrode ( 15 ) are part of an active matrix, enabling the display to be used also in a fast video mode. Thus, a bistable liquid crystal display device is provided which has two drive modes, a low frequency mode, (first drive mode, also called “bistable mode”, “passive mode” or “high voltage mode”) for applications requiring slower switching times and lower power consumption and a high frequency mode (second drive mode, also called “active mode, “active matrix drive mode” or “fast video mode”) for grey scale images and video applications.

The invention relates to a liquid crystal display device comprising anematic liquid crystal material between a first substrate and a secondsubstrate, at least one substrate being provided with electrodes, whichdefine picture elements, the device comprising driving means for drivingthe picture elements in a first mode of driving between two stablestates.

Liquid crystal display effects, based on bistability of a nematic liquidcrystal material, are well known. One example is the supertwist nematiceffect, showing two stable states, which is used in many displayapplications, ranging from mobile phones to laptop computers. Otherbistable electro-optical effects have been described, for instance, byDozov et al. (“Recent Improvements of Bistable nematic Displays Switchedby Anchoring Breaking”, SID 2001, pages 224-7) and by Guo etal.(Three-terminal bistable twisted nematic liquid crystal displays”,Applied Physics Letters, Vol. 77, No 23, pages 3716-3718).

Bistable liquid crystal displays have a very low power consumption ifthe update frequencies are low. This makes them very suitable forapplications in mobile devices like electronic books. However, in theseapplications a growing need exists for the possibility to show imageshaving color, grey-scales and video content.

In general, it is not very well possible to fulfill these needs with thebistable electro-optical effects. In general, they are restricted toonly a few color applications and switching times (of the order of 300ms) which are too slow for video applications (which require switchingtimes of the order of 10-20 ms).

It is one of the objects of the invention to overcome these drawbacks byproviding a bistable liquid crystal display device having two drivemodes, of driving viz. a low frequency mode for applications requiringslower switching times and a high frequency mode for e. g. videoapplications.

It is another object of the invention further has as one of its objectsto provide a bistable liquid crystal display device which is alsosuitable for video applications or other applications which require ahigh frequency mode.

To this end, a liquid crystal display device according to the inventioncomprises driving means for driving the picture elements in a first modeof driving between two stable states, liquid crystal molecules in thestable states having different twist angles, viewed from one substrateto another in said first drive mode and driving means for driving thepicture elements in a second mode of driving between two opticalextremes of the picture elements, the difference in twist angles of theliquid crystal molecules, viewed from one substrate to another in saidsecond drive mode, being substantially constant.

An optimal extreme in this connection may refer to an extreme in avoltage vs. light transmission curve, a voltage vs. light reflectioncurve or a voltage vs. light adsorption curve.

The invention is based on the insight that, by preventing the moleculesfrom twisting too much in the second drive mode, the molecules switchfaster between intermediate states, which states may be determined by avoltage provided by a switching device. Too much twisting can beprevented in the second mode by limiting the voltages on the pictureelements to a maximum value at which substantially switching to theother bistable states is initiated. In this second mode, the differencein twist angles of the liquid crystal molecules, viewed from onesubstrate to another in said second mode, may be different from thedifference in twist angles in one of the stable states due to surfaceeffects or due to the kind of driving (e.g. due to lateral switchingfields)

In the first drive mode, generally higher switching voltages are needed.If active matrix driving is used (e.g. a TFT—matrix) in the second mode,the active matrix will generally not be capable of providing the highvoltages required for the first drive mode, and in addition, it will benecessary to modulate said first drive mode for time periods which maynot correspond to complete frame periods. In many cases, shorter pulseswill be required to set up bistable states in said first drive mode. Inone possible solution, the display is provided with two complete drivesystems, an active matrix and a passive (if necessary with reset) drivesystem, for example by providing strip-shaped electrodes on the secondsubstrate, which could be driven from a separate passive matrix typedriving chip. In the active matrix mode, the electrodes on the secondsubstrate would be shorted (virtually) and driven with the adequatesignals. In addition, the columns could also be attached to a second(higher voltage) chip if the normal column driver delivers insufficientvoltage for passive (reset) driving.

In a first embodiment the liquid crystal display device has comb-shapedelectrodes for each picture element and a further electrode on the firstsubstrate.

The driving means for driving the picture elements in the first drivemode in this embodiment provide driving pulses to the comb-shapedelectrodes on the first substrate and driving pulses to the furtherelectrode.

If the second substrate is provided with strip-shaped electrodes, theliquid crystal display device now has driving means for driving thepicture elements in the first drive mode and provide driving pulses tothe comb-shaped electrodes on the first substrate and driving pulses tothe further strip-shaped electrode.

In the first drive mode, generally higher voltages are required than inthe active matrix mode. To prevent using a high voltage driver, drivingmeans comprising means for bringing the picture element to a definedstate may be introduced, so a commercially available (low voltage)driver device can be used.

To this end, one embodiment of the liquid crystal display devicecomprises two row electrodes for each row of picture electrodes andcolumn electrodes on the first substrate, the switching elementcomprising at least two thin-film transistors, each thin-film transistorbeing selectable by one of said two row electrodes.

In another embodiment, a pulse for bringing the picture element to thedefined state is produced by capacitive coupling.

The driving speed in the second (active matrix) is increased if thepicture element at the first substrate comprises at least threeelectrodes, the driving means comprising means for generating electricfields in different (preferably substantially perpendicular) directions.

If necessary, the second (active matrix) drive mode can be supplied to abistable liquid crystal display device without coupling it to a first(passive) mode. The device then comprises driving means for driving thepicture elements in a mode of driving between two optical extremes ofthe picture elements, the difference in twist angles of the liquidcrystal molecules, viewed from one substrate to another in said drivemode, being substantially constant These and other aspects of theinvention are apparent from and will be elucidated with reference to theembodiments described hereinafter.

In the drawings:

FIG. 1 is an electric circuit diagram of the display device,

FIG. 2 is a cross-section of a display cell of a device according to theinvention,

FIG. 3 is a plan view of a picture electrode in a display cell of adevice according to the invention,

FIG. 4 is a cross-section taken on the line IV-IV in FIG. 2,

FIG. 5 shows the response of such a display device,

FIG. 6 shows another effect to which the invention is applicable,

FIGS. 7, 8 are plan views of other picture electrodes in a display cellof devices according to the invention,

FIG. 9 is a plan view of a picture electrode in a display cell ofanother device according to the invention, together with a drivingpulse,

FIG. 10 shows part of a device for generating the column pulses, while

FIGS. 11, 12 are plan views of picture electrodes in display cells ofother devices according to the invention, together with the drivingpulses, and

FIG. 13 is a cross-section of another display cell.

The Figures are diagrammatic and not drawn to scale; corresponding partsare generally denoted by the same reference numerals.

FIG. 1 is an electric equivalent circuit diagram of a part of a displaydevice 1 to which the invention is applicable. It comprises a matrix ofpixels 18 at the area of crossings of row or selection electrodes 17 andcolumn or data electrodes 6. The row electrodes are consecutivelyselected by means of a row driver 16, while the column electrodes areprovided with data via a data register 5. To this end, incoming data 8are first processed, if necessary, in a processor 10. Mutualsynchronization between the row driver 16 and the data register 5 takesplace via drive lines 7.

In one drive mode, called the “active mode” signals coming from the rowdriver 16 select the picture electrodes via thin-film transistors (TFTs)19 whose gate electrodes 20 are electrically connected to the rowelectrodes 17 and the source electrodes 21 are electrically connected tothe column electrodes. The signal which is present at the columnelectrode 6 is transferred via the TFT to a picture electrode of a pixel18 coupled to the drain electrode 22. The other picture electrodes areconnected to, for example, one (or more) common counter electrode(s) 15.

FIG. 2 is a cross-section of a part of a liquid crystal material 2 whichis present between two substrates 3, 4 of, for example, glass or(flexible) synthetic material, provided with (ITO or metal) pictureelectrodes (not shown) and a counter electrode (not shown),respectively. As described by Guo et al. (Three-terminal bistabletwisted nematic liquid crystal displays”, Applied Physics Letters, Vol.77, No 23, pages 3716-3718), if bistable liquid crystal displays areused, switching occurs between two bistable states φ, φ+π (schematicallyshown in FIG. 2 a, in which the (directors 27 of the) liquid crystalmolecules either have a twist (right side) or no twist (left side)).

Switching between the two bistable states is obtained by pulses ofrather high voltages (of the order of 15-30 V), the threshold voltagefor switching being rather high. However, the inventors have found that,by using voltages below said threshold voltage, a drive mode of fastswitching between grey-levels in a grey-scale between two transmissionextremes is possible. One of the two states may be a first state inwhich substantially all (directors 27 of the) liquid crystal moleculeshave an orientation parallel to the first substrate 3, schematicallyshown in FIG. 2 b, (left side). This state is comparable to one of thetwo bistable states. In the other transmission extreme, tilting of the(directors 27 of the) liquid crystal molecules occurs, introducingpolarization change. Between crossed polarisers, in this example, a darkpixel is obtained (left side, no voltage) or a white or gray pixel isobtained (right side), dependent on the voltage used. Sincesubstantially no twisting occurs, said driving between two transmissionextremes can be much faster than the driving between the bistablestates.

The (directors 27 of the) liquid crystal molecules do not necessarilyhave to tilt. Also a twisting effect, comparable to the “in planeswitching” effect is possible. A picture element using this effect isshown in FIGS. 3, 4. FIG. 3 is a plan view and FIG. 4 is a cross-sectiontaken on the line IV-IV in FIG. 3 of a part of a liquid crystal device.Liquid crystal material 2 is present between two substrates 3, 4 of, forexample, glass or (flexible) synthetic material, provided with (ITO ormetal) comb-shaped picture electrodes 14 and a counter electrode 15,respectively. The combshape of the picture electrodes 14 introducesfringe fields, needed for the switching between the bistable states. Thedevice also comprises orientation layers 13, which orient the liquidcrystal material on the inner walls of the substrates. Moreover, thedevice comprises a polarizer (not shown) and a (mutually perpendicularlycrossed) analyzer. In this case, the liquid crystal material is a(twisted) nematic material having a positive dielectric anisotropy. Thedevice further comprises (ITO) ground plane electrodes 12, isolated fromthe picture electrodes 14 by an isolation layer 11.

In a first drive mode, the “bistable mode” or “passive mode”, signalsV_(comb) (voltages indicated by pattern 30 in FIG. 4 b) on comb-shapedpicture electrodes 14 and V_(ground) (voltages indicated by pattern 31in FIG. 4 b) on ground plane electrodes 12, are used to switch from adark state to a light state and to switch from a light state to a darkstate, respectively.

FIG. 3 shows row or selection electrodes 17 and column or dataelectrodes 6. As described above, in the second drive mode, called the“active mode”, the picture electrodes are selected via (schematicallyshown) thin-film transistors (TFTs) 19 whose gate electrodes 20 areelectrically connected to the row electrodes 17, while the sourceelectrodes 21 are electrically connected to the column electrodes. Thesignal that is present at the column electrode 6 is transferred via theTFT to the picture electrode 14. Dependent on the voltages used, sometwisting and tilting is induced in the liquid crystal molecules,defining a certain grey value. The signal on the picture element,however, should not be so high that switching to the other bistablestate may occur.

FIG. 5 shows one way of switching in the fast “active mode”. First (apart of) the display is reset by a sufficiently high voltage (e.g. 40V), between counter electrode 15 and the electrodes 12, 14, comparableto FIG. 4(the pulse for resetting may be much shorter than a frameperiod). Subsequently, during a first frame period t_(fl) the voltageV_(count) is held at e.g. 0V (voltage pattern 32) while both the pictureelectrode 14 and ground plane electrode 12 are given a high voltage(V_(comb), V_(ground), voltage patterns 30, 31). If necessary, thepicture electrode 14 and ground plane electrode 12 may be given the samevoltage e.g. by introducing an extra witch. During line selections inthe subsequent frame periods, (lower) voltages between the counterelectrode 15 and the electrodes 12, 14 define grey values. The resultingtransmission curve is indicated by reference numeral 33.

Said reset voltage as well as the “bistable mode” or “passive mode”signals in the first drive mode, as shown in FIG. 4 b are applied viasaid thin-film transistors (TFTs) 19.

Similar remarks apply to a device, based on the effect described inDozov et al. (“Recent Improvements of Bistable Nematic Displays Switchedby Anchoring Breaking”, SID 2001, pages 224-7). FIG. 6 schematicallyshows again two bistable states φ, φ+π, in which the (directors 27 ofthe) liquid crystal molecules either have a twist (right side, T state)or no twist (left side, U state). A pulse pattern 35 introducesswitching from the U state to the T state, whereas a (voltage) pulsepattern 36 introduces switching from the T state to the U state. Usingthis effect in a passive matrix, multiplex driving is possible with linevoltages of up to 16 V and column voltages of ±2 V.

The display device of FIG. 1 also comprises an auxiliary capacitor 23 atthe location of each pixel. The auxiliary capacitor may be connectedbetween the common point of the drain electrode 22 and the pixel in agiven row of pixels at one end, and the row electrode of the previousrow of pixels at the other end; other configurations are alternativelypossible, for example, between said common point and the next row ofpixels, or, as shown in FIG. 1, between this point and an extra rowelectrode 17 for a fixed (or variable) voltage.

In this embodiment, the display device comprises separate electrodes 15,but these electrodes may also be provided as a single common electrode(counter electrode). As will be discussed later, these extracapacitances may be involved in generating the high voltage pulses, asneeded for either resetting (part of) the display or generating the highvoltage pulses for bistable addressing (first mode).

In FIG. 7, the matrix column driver 5 is provided with drivers providinga sufficiently high voltage V_(reset) (pulse 40) for the reset andbistable addressing (first mode). The necessary duration of the voltagepulse is obtained by selecting a voltage V_(t) (pulse 42) during a firstline time t₁₁ via TFT transistor 19, selected via row electrode 17 anddriving the voltage back to zero after a defined time period t_(r) via asecond TFT transistor 19′, selected via an extra row electrode 17′during a second line time t₁₂. Drain 21′ of transistor 19′ is coupled(capacitively or direct) to a voltage line 34, in this example ground.It is alternatively possible to select TFT 19 again after a defined timeperiod t_(r) to reset every pixel to a reference voltage (e.g. zero)(FIG. 8). The reset pulse can scan from top to bottom across the display(row at a time) or be applied to the complete display.

FIG. 9 shows an embodiment in which low voltage column drivers 5 do notproduce a sufficiently high voltage to enable the reset driving asdescribed above. Part of a display, similar to the display of FIG. 1 isschematically shown, the pixel 18 being indicated by its capacitanceC_(1c). The storage capacitor 23, indicated by its capacitanceC_(store), parallel to the pixel is used to couple (additional) voltageto the pixel. In this embodiment, a separate selection line 17′ is usedfor coupling the voltage to the pixel. First, the pixel is driven attime t₁ with the maximum voltage V_(cmax), available from the columndriver, after which the additional voltage is added by capacitivecoupling at time t₂ (full line in FIG. 9 b). The magnitude of theadditional voltage is determined by the applied voltage V_(cap) onselection line 17′ and the ratio of the pixel to storage capacitors (alarge storage capacitance is preferred), whilst the timing intervalt₂−t₃ is determined by the pulse on the storage capacitance line. Thisadditional voltage is determined byΔV=V_(cap).(C_(store)/C_(1c)+C_(store))). Pixels which do not requirethe high select voltage are selectable by first driving these pixels toeither the lowest possible column driver voltage (i.e. 0V in thisexample—see dash-dot line in FIG. 9 b) or even to a maximum voltage ofopposite polarity (dotted line in FIG. 9 b) just before applying thecapacitive coupling. In this way, they will not reach the voltagerequired to switch, and will remain in the initially defined state. Thestorage capacitor may also be connected to the next or previous row 17as shown in FIG. 1.

In the embodiment of FIG. 10, column lines 6 can be directly connectedto a high voltage line 44. In this embodiment, the high voltage is madeavailable in the column driver IC as a single high voltage (power) line44. A switch 45 in each (IC) output buffer 46 is used to connect thecolumn lines 6 to either the high voltage line 44 or to the normal lowvoltage (grey scale) output driving circuit 47. To change the bistablestate, the pixels are driven to the high voltage and held for a longerperiod (e.g. a full fame period, or integral thereof). The selectionswitch is connected to the high voltage line (see column 2 in FIG. 10).Pixels which do not require their state to change will be connecteddirectly to the low voltage output driving circuit 47 (columns 1 and 3in FIG. 10).

When driving the display in the bistable mode, the high voltage linemust be activated. To reduce power dissipation, it is advisable todisable the high voltage line (cut off the high voltage power supply)whilst the display is operating in the normal active matrix mode.Preferably the power supply voltage is thus changed, depending upon thedisplay mode used.

An embodiment to obtain the pulses as shown in FIG. 6 (for a device asdescribed by Dozov et al. (“Recent Improvements of Bistable NematicDisplays Switched by Anchoring Breaking”, SID 2001, pages 224-7) isshown in FIG. 11. Selection is carried out by applying a high voltage(e.g. 15V), and either returning to 0V in a single step to the twistedstate (pulse 48 in FIG. 11 b) or with a pause at an intermediate voltage(to the non-twisted state pulse 49 in FIG. 11 b).

In this embodiment, using low voltage column drivers, the pixels areconnected (via a TFT 19′) to a high voltage select line 17′, madeavailable from the row driver 16. In this direct drive embodiment, itwould be possible to address all pixels in a row to high voltage (fromV_(select)) during a first frame at t₁, and then carry out a selectionto either 0 V (pulse 48 in FIG. 11 b), or to an intermediate voltage(e.g. 5 V) during a second frame at t₂ ( pulse 49 in FIG. 11 b), beforereturning all pixels to ₀V in a third frame. These devices may also bedriven faster. The first part of the signal (above threshold, asindicated by t₁−t₂) could be, for example, at least 50 μs, while thesecond part of the signal may have any duration between 50 μs and aframe time. Next to selecting the entire display before the second gatepulse is applied, it is possible to make a line after line selection tobring separate lines successively to a defined state.

FIG. 12 shows an embodiment in which (an entire row of) pixels aredriven to the high voltage using a capacitive coupling from a separatecapacitor line, in a similar way as described with respect to FIG. 9.Again all pixels are addressed to the maximum pixel voltage V_(max) andthen to V_(select). This select voltage is again obtained by adding thecapacitive voltage ΔV (e.g. one line time after driving the pixel topixel voltage V_(max)). In this case, the voltage is held high untiljust before the following address period (in the following frame) whenthe capacitive coupling returns to zero. At the next address period,pixels which are to be twisted are addressed to 0V (fixed line in FIG.12 b, comparable to line 48 in FIG. 11 b) directly after the capacitivevoltage step (within a few microseconds). This will appear to the LC asif it is directly returned to 0V, and the twisted state will be created.Pixels, which should not be twisted, should be addressed to anintermediate pixel voltage (e.g. pixel voltage V_(max)) and held at saidvoltage for a sufficiently long period before being returned to 0V in alater frame (dashed line in FIG. 12 b, comparable to line 49 in FIG. 11b).

The driving speed, especially in the “active” mode, when the moleculestilt between different positions, according to the grey value, isenhanced by “dynamic driving”. One example is shown in FIG. 13 in whichthe picture electrode 14 has been split up into sub-electrodes 14 ^(a),14 ^(b) which are driven by separate column lines and TFTs (not shown).Dependent on the voltages on the datalines and on the counterelectrode15, electric fields are introduced between these electrodes. Electrodes14 are suited for generating electric fields 51 parallel to thesubstrates 3, 4, whereas these electrodes together with counterelectrode15 are suited for generating electric fields 52 normal to the substrates3, 4. By suitable choices of voltages during switching, the torque whichthe resulting electric field exercises on the (directors of the) liquidcrystal molecules is optimized both during switching on and duringswitching off in the “active” mode.

The protective scope of the invention is not limited to the embodimentsdescribed. For instance, the pulse shape 36 as described in FIG. 6 maybe different (linearly or exponentially decreasing in the second part),for instance, by means of a (controlled) resistor to a (fixed) voltage,if necessary, controlled by an extra switch. The invention resides ineach and every novel characteristic feature and each and everycombination of characteristic features. Reference numerals in the claimsdo not limit their protective scope. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements other than thosestated in the claims. Use of the article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.

1. A liquid crystal display device comprising a nematic liquid crystalmaterial between a first substrate and a second substrate, at least onesubstrate being provided with electrodes, which define picture elements,the device comprising driving means for driving the picture elements ina first mode of driving between two stable states, liquid crystalmolecules in the stable states having different twist angles, viewedfrom one substrate to another in said first drive mode, and drivingmeans for driving the picture elements in a second mode of drivingbetween two optical extremes of the picture elements, the difference intwist angles of the liquid crystal molecules, viewed from one substrateto another in said second drive mode, being substantially constant.
 2. Aliquid crystal display device as claimed in claim 1, wherein thedifference in twist angles of the liquid crystal molecules, viewed fromone substrate to another in said second mode, is different from thedifference in twist angles in said first drive mode.
 3. A liquid crystaldisplay device as claimed in claim 1, wherein in the second mode thevoltages on the picture elements have a maximum value at which noswitching to the first mode occurs.
 4. A liquid crystal display deviceas claimed in claim 1, comprising, on the first substrate, a switchingelement between a driving electrode and a picture electrode.
 5. A liquidcrystal display device as claimed in claim 4, comprising row electrodesand column electrodes on the first substrate, the switching elementbeing a thin-film transistor.
 6. A liquid crystal display device asclaimed in claim 4, comprising strip-shaped electrodes on the secondsubstrate.
 7. A liquid crystal display device as claimed in claim 1,having for each picture element comb-shaped electrodes and a furtherelectrode on the first substrate.
 8. A liquid crystal display device asclaimed in claim 6, comprising strip-shaped electrodes on the secondsubstrate, the driving means for driving the picture elements in a firstdrive mode providing driving pulses to the comb-shaped electrodes on thefirst substrate and driving pulses to the strip-shaped electrodes on thesecond substrate.
 9. A liquid crystal display device as claimed in claim7, wherein the driving means for driving the picture elements in a firstdrive mode provide driving pulses to the comb-shaped electrodes on thefirst substrate and driving pulses to the further electrode.
 10. Aliquid crystal display device as claimed in claim 1, wherein the pictureelement at the first substrate comprise at least two electrodes, thedriving means comprising means for generating electric fields in twodifferent directions.
 11. A liquid crystal display device as claimed inclaim 10, wherein in which the electric fields have substantiallyperpendicular directions.
 12. A liquid crystal display device as claimedin claim 1, wherein the driving means for driving in the first modecomprise means for bringing the picture element to a defined state. 13.A liquid crystal display device as claimed in claim 12, comprising rowelectrodes and column electrodes on the first substrate, the switchingelement comprising a thin-film transistor, the driving means comprisingmeans for producing a pulse for bringing the picture element to thedefined state.
 14. A liquid crystal display device as claimed in claim12, comprising two row electrodes for each row of picture electrodes andcolumn electrodes on the first substrate, the switching elementcomprising at least two thin-film transistors, each thin-film transistorbeing selectable by one of said two row electrodes.
 15. A liquid crystaldisplay device as claimed in claim 12, wherein a pulse for bringing thepicture element to the defined state is produced by capacitive coupling.16. A liquid crystal display device as claimed in claim 1, wherein thedifference in twist angles, viewed from one substrate to another in saidfirst drive mode, is substantially 180 degrees or a multiple of 180degrees.
 17. A liquid crystal display device comprising a nematic liquidcrystal material between a first substrate and a second substrate, atleast one substrate being provided with electrodes, which define pictureelements, the liquid crystal molecules being able to obtain two stablestates having different twist angles, viewed from one substrate toanother, the device comprising driving means for driving the pictureelements in a mode of driving between two optical extremes of thepicture elements, the difference in twist angles of the liquid crystalmolecules, viewed from one substrate to another in said drive mode,being substantially constant.
 18. A liquid crystal display device asclaimed in claim 11, wherein the voltages on the picture elements have amaximum value at which no switching to another stable state occurs.